Why does this code run fine on my Laptop (OSX 10.9), but segfault on server (Linux)?

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef char String[256];

typedef struct LinkedListNode {
String Name;
struct LinkedListNode * Next;
struct LinkedListNode * Friend;
} Node;

typedef Node * NodePointer;


NodePointer InputData(NodePointer Head) {

    String PersonName;
    NodePointer Person;
    int StringLength;
    String FormattedName;
do {    
        printf("Enter nation name :");
        fgets(PersonName,256,stdin);
        if (PersonName[0] != '\n') {
            Person = (NodePointer)malloc(sizeof(NodePointer));
            //copy all except trailing \n
            strncpy(Person->Name, PersonName, strlen(PersonName)-1);
            Person->Next = Head;
            Person->Friend = NULL;
            Head = Person;
        }
    } while (strcmp(PersonName, "\n"));
return Head;
}

void InputAllies(NodePointer Head) {
    String AllyName;
    NodePointer Ally;
    String FormattedName;
    do {    
            printf("Enter best ally for %s :", Head->Name);
            fgets(AllyName,256,stdin);  
            if (AllyName[0] != '\n') {
                Ally = (NodePointer)malloc(sizeof(NodePointer));
                //copy all except trailing \n
                strncpy(Ally->Name, AllyName, strlen(AllyName)-1);
                Head->Friend = Ally;
                Head = Head->Next;
            }
        } while (Head != NULL);
}

The part that segfaults specifically is the InputAllies() function, and only with lists with 5 or more elements. I really have no idea what's going on, but I assume it has something to do with the size of my strings. Lowering the size of the typedef String causes a segfault after only 3 elements.

Answer 1

I think that in function InputData , just after the strncpy , you are forgetting to add the final '\\0' to the string Person->Name . The problem is when you print the string. On your Mac, it seems that there is a null character that happens to be magically in the right place; on the Linux server, you're not so lucky.

Copying from the manual :

The strcpy() function copies the string pointed to by src , including the terminating null byte ( '\\0' ), to the buffer pointed to by dest . The strings may not overlap, and the destination string dest must be large enough to receive the copy.

The strncpy() function is similar, except that at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src , the string placed in dest will not be null-terminated.

So your problem is the last sentence, and it will happen only when your source string cannot fit in the first n bytes of your target string. I think that's exactly your case here, as you're setting n to be less than the strlen of your source string. In any case, there may be other bugs in your code too. Check with a debugger or valgrind.

BTW, I don't understand why you want strncpy there, as your source and target strings are necessarily of the same length. Why not use strcpy instead?